1. Field of the Invention
The present invention relates to a CRT (Cathode Ray Tube) line scan circuit, and more specifically to a circuit enabling the use of the same geometry correction set-point whatever the scan frequency used. The invention is especially useful in computer monitors.
2. Discussion of the Related Art
Conventional scan circuits are described in several reference works, among which xe2x80x9cNew Handbook of Color Televisionxe2x80x9d, Chiron ed., volume 2, pages 197-207, 1981.
FIG. 1 shows a conventional scan circuit of the type which also generates the very-high voltage (THT) for the CRT. Between a node P and the ground are arranged, connected in parallel, a switch T, an antiparallel diode D, and a retrace capacitor Cr. The scan coil, Ld, is series-connected with a capacitor Cs between node P and ground. Capacitor Cs has a large value with respect to capacitor Cr.
Further, the primary winding Lp of a step-up transformer 10 is series-connected with a supply voltage source E between node P and ground. The very-high voltage THT is taken across the secondary winding Ls of transformer 10, rectified by a diode D1 and filtered by a capacitor C1.
FIG. 2 shows the shape of the deviation current flowing through coil Ld and the corresponding voltage taken from node P. Generally, the inductance of primary winding Lp of step-up transformer 10 is so high that it can be considered as an infinite impedance. Similarly, capacitor Cs has a value such that it can be considered as a low impedance voltage source providing a voltage substantially equal to supply voltage E (actually, capacitor Cs is maintained charged to value E via supply source E and windings Lp and Ld).
At a time t0, switch T is closed and the current through coil Ld is zero. Coil Ld is connected across capacitor Cs providing substantially constant voltage E. The current through capacitor Ld increases linearly with a slope E/Ld.
At a time t1, switch T is open. The current through coil Ld has reached a peak value Ip equal to Exc2x7t1/Ld (assuming that t0=0). Coil Ld is now connected in an L-C oscillating circuit with capacitor Cr, whereby the current through the coil varies as a sinusoid to reach a value-Ip at a time t2. Interval t1-t2 corresponds to a half-sinusoid of time constant {square root over ((Ldxc2x7Cr))}. It further corresponds to a line retrace which is of short duration with respect to the line scan duration.
During interval t1-t2, the voltage at node P varies according to a sinusoid halfwave of amplitude Up=Exc2x7t1/{square root over ((Ldxc2x7Cr))}. This halfwave is applied to the primary winding of step-up transformer 10 to be converted into a very-high voltage.
At time t2, the voltage on node P tends to become negative to start a negative sine halfwave. Then, diode D turns on and takes all the current coming from coil Ld. Coil Ld is connected again across the voltage source formed by capacitor Cs, so that the current starts increasing linearly with a slope E/Ld to cross value 0 at a time txe2x80x20 where the cycle starts again. It should be noted that interval t2-txe2x80x20 has the same duration t1 as interval t0-t1.
In some applications, especially in computer monitors, the scan frequency, that is, time t1, should be variable. In the circuit of FIG. 1, it should be noted that the peak scan current Ip and the peak voltage Up are inversely proportional to the scan frequency. Now, modifying peak current Ip is out of the question, since it determines the horizontal scan amplitude, and the same applies to peak voltage Up, since it determines the high voltage which has to be maintained substantially constant. At first sight, it would be enough to vary supply voltage E proportionally to the frequency to maintain constant the scan amplitude and the high voltage.
However, the image width should be adjustable, and it must be possible to make geometry corrections. For example, if the scan current amplitude was maintained constant over the image height, the image edges would have a parabolic shape, which defect is corrected by modulating the amplitude of the scan current in a complementary parabolic way.
In other words, for a fixed scan frequency, the scan current amplitude should be modulated without modifying the value of the high voltage.
The circuit of FIG. 1 has thus been improved in the prior art to achieve this.
FIG. 3 shows such a circuit, called a xe2x80x9cdiode modulatorxe2x80x9d. It includes the same elements as the circuit of FIG. 1, designated by the same references. Two oscillating circuits having the same time constants are connected in series across switch T, which remains connected between node P and ground. One of the oscillating circuits includes elements Ld, Cs, Cr, and D described in relation with FIG. 1. The second oscillating circuit includes an inductor Lm, two capacitors Cm and Crm, and a diode Dm, respectively homologous to elements Ld, Cs, Cr, and D. However, inductor Lm has no influence upon the horizontal deviation. A modulation voltage Vg is forced across capacitor Cm. This modulation voltage is intended for correcting the scan current without having any effect upon the value of the high voltage.
Each of the oscillating circuits operates independently and similarly during a scan cycle. Indeed, when switch T is closed, inductor Lm is connected across capacitor Cm, as illustrated, via diode D and switch T, while scan inductor Ld is connected across capacitor Cs, as illustrated, via diode Dm and switch T. When switch T is open, the current flows in each of the oscillating circuits as described for the single oscillating circuit of FIG. 1.
With this configuration, peak scan current Ip can be expressed as (Exe2x88x92Vg)xc2x7t1/Ld. Indeed, the voltage across capacitor Cs settles at value Exe2x88x92Vg and not at value E. Thus, the scan current amplitude is adjustable by acting upon modulation voltage Vg. Further, the peak voltage appearing on node P is equal to the sum of the peak voltages of the two oscillating circuits, that is:
Ip=(Exe2x88x92Vg)xc2x7t1/{square root over ((Ldxc2x7Cr))}+Vgxc2x7t1/{square root over ((Lmxc2x7Crm))}
This current is equal to Exc2x7t1/{square root over ((Ldxc2x7Cr))}, since time constants Ldxc2x7Cr and Lmxc2x7Crm are chosen to be equal. Thus, the value of the high voltage is independent from modulation voltage Vg. However, if it is desired, in the circuit of FIG. 3, to keep the features of the displayed image when the scan frequency varies, not only supply voltage E, but also modulation voltage Vg, have to be modified proportionally to the frequency. Indeed, the scan current amplitude is proportional to Exe2x88x92Vg.
Supply voltage E can easily be varied according to the scan frequency, since this voltage is generally generated by a D.C./D.C. converter synchronized on the scan frequency and operating in a mode such that it generates a voltage proportional to its operating frequency.
However, to make modulation voltage Vg proportional to the frequency, a frequency-to-voltage converter and a multiplier which multiplies a set-point voltage independent from the frequency by the output of the frequency-to-voltage converter are generally used. This solution is complex and inaccurate.
An object of the present invention is to provide a line scan circuit which enables the modulation voltage to be made proportional to the frequency in a particularly simple and accurate manner.
To achieve this and other objects, the present invention provides a line scan circuit for a CRT, including, in series across a switch, two oscillating circuits having the same time constant, each including, in parallel, a capacitor, a diode connected in antiparallel, and a series association of an inductor and of a voltage source, the inductor of a first one of the oscillating circuits being a scan coil of the CRT; and means for adjusting the voltage source of the second oscillating circuit. The adjusting means include an amplifier receiving a set-point voltage and using, as a feedback, a voltage sampled from one of the oscillating circuits.
According to an embodiment of the present invention, the feedback voltage is the peak voltage taken across the second oscillating circuit.
According to an embodiment of the present invention, the feedback voltage is a voltage taken between two series-connected capacitors forming the capacitor of the second oscillating circuit.
According to an embodiment of the present invention, the amplifier includes an operational amplifier, the inverting input of which receives a negative set-point voltage via a first resistor and the feedback voltage via a second resistor.
According to an embodiment of the present invention, the circuit includes a peak detector with a diode connected in series with a capacitor.
According to an embodiment of the present invention, the voltage source of the first oscillating circuit is formed of a capacitor maintained charged by a supply voltage source connected in series with the primary winding of a step-up transformer across the switch.